It's only because that's the only stat other classes know to look for in a tank (in addition to health and armor). They've been conditioned from TBC to ask for 490def, and they automatically look for new min. defense requirement, which is 540. They also want to know if you've put in the effort to become a tank.
Originally Posted by justatank
The "valid reason" is simply because they want to make sure they take the steps necessary to increase the chances of their PUG's success. If you were in their position, wouldn't you do the same?
It's the same thing when looking for DPS for a heroic; most people would ask "what's your DPS?" They won't ask "what's your spec? Do you know how to play your class? Which stats are you stacking?"
Yes, Defense is the better option for uncrittability than resilience because it gives you more. And of course you can stack resilience to "patch" the lack of Defense in your gear.
Next time people ask you "what's your def?", just tell them like it is. Something like "I'm 530, but I got the resilience to make me uncrittable." If they don't know what you're talking about, then too bad. If they're willing to listen, just give them a bit of a primer about uncrittability and how resilience works. I don't see what's the problem with that.
I've got more plate than your grandma's cupboard.